1. Field of the Invention
The present invention relates generally to a transformer of double-insulated type which meets the Standard of IEC (International Electrotechnical Commission), and in particular to a bobbin case of the transformer in which primary and secondary windings are accomodated. The present invention also relates to a method of assembling the double insulated transformer provided with the bobbin case.
2. Related Art Description
A conventional transformer of double-insulated structure comprises a bobbin case into which the primary and secondary winding assemblies are inserted. The bobbin case is comprised of electrically insulated two parts, one for receiving the primary winding assembly and the other for receiving the secondary winding assembly. These two parts are formed integrally by insulating material such as nylons.
A typical bobbin case, primary winding assembly and secondary winding assembly are illustrated in FIGS. 9A to 9C, 10 and 11, respectively. As shown in FIGS. 9A to 9C, the bobbin 1 comprises a first portion 2 for receiving a primary winding assembly and a second portion 3 for receiving a secondary winding assembly. These portions are formed integrally adjacent to each other and are electrically separated by a partition wall 4 therebetween. The second portion 3 has a rectangular recess 3A opened laterally into which the secondary winding assembly 5 shown in FIG. 11 is inserted from the lateral direction. The secondary winding assembly 5 is constituted by a bobbin 5A and a secondary winding 5B around the bobbin 5A. The bobbin 5A is formed with a center hole 5C into which a core is to be inserted. Likewise, the primary winding assembly 6 is constituted by a bobbin 6A having a center hole 6C and a primary winding 6B arround the bobbin 6A, as shown FIG. 10.
While, the first portion 2 has a rectangular recess 2A which is defined by the partition wall 4 and a peripheral wall 2B surrounding the three sides of the partition wall 4. The peripheral wall 2B is formed with two rims 2C and 2D extending inwardly from the vertical edges of the wall 2B. The rims 2C, 2D are provided for obtaining a creeping distance between the primary winding 6A and the core inserted into the ceter holes 5C and 6C. Since these rims 2C and 2D are provided, the primary winding assembly 6 is inserted into the recess 2A through a downward opening 2E of the recess 2A along the vertical direction shown by arrow A in FIG. 12, whereas the secondary winding assembly 5 is inserted into the recess 3A along the horizontal direction shown by arrow B in FIG. 12.
As described above, the directions in which the primary and secondary winding assemblies 5, 6 are inserted into the bobbin case 1 are different from each other, so that whenever winding assemblies are inserted into the bobbin case, it is necessary to determine as to which type the winding assembly to be inserted is and from which direction the winding assembly in question should be inserted into the bobbin case. This causes the inserting operation of the winding assemblies to be complicated and inefficient. In addition, if the inserting operation is intended to carry out automatically, such an assembling device tends to have a complicated mechanism to perform the inserting operations in the different directions.
Furthermore, after the primary winding assembly 6 is inserted, the bobbin case 1 must be maintained such that the opening 2E of the recess 2A is oriented in the upward direction so as not to fall the inserted primary winding assembly 6 from the recess 2A. The core is then inserted into the center holes 5C and 6C with maintaining the bobbin case in this condition. This operation is not easy or efficiently to carry out. In addition, the primary winding assembly 6 after inserted into the recess 2A is exposed at its winding portion 6B through the opening 2E. Therefore, the exposed winding portion must be covered thereafter with insulating tape and the like. This means that additional taping process is needed to assemble the transformer.
As shown in FIG. 12, an E-shaped core assembly 7 is then inserted into the center holes of the assembly 8 of the bobbin case, and the windings and in turn the a flat core assembly 9 is attached to the E-shaped core assembly 7. The resulting assembly is then varnished in order to improve insulation properties of the obtained transformer and to reduce noise in operation of the transformer. The varnish is conducted by means of dipping the transformer into a varnish bath. As shown in FIG. 13, a hanging member 11 is arranged horizontally above a varnish bath 12. The hanging member 11 is provided with several sets of hook members 13A, 13B, hanging therefrom. Each of transformers 10 is hanged on each set of the hook members, and the hanging member 11 is then lowered to dip the hanged transformers 10 into the varnish bath 12. After a predetermined period of time the dipped transformers are pulled out of the bath 12.
This varnishing process is not always employed. For example, if the transformer has no suitable portions the hook members are to be engaged with, it cannot be hanged on the hanging member. In such a case, the varnishing is carried out by placing the transformer in a net and dipping the net into the varnish bath. However, this process is not so efficient as the hanging process.
While, as shown in FIG. 14, the primary and secondary winding assemblies 6 and 5 are provided at their lower ends with external terminals 15, which are joined to connecting pins 16 by soldering. For the purpose of protecting the soldering portions 17 of the respective pins, the conventional bobbin 5 or 6 is provided integrally with protecting legs 18 projecting downwardly from the lower end of the bobbin. Thus, a mold for forming the bobbin with the protecting legs becomes rather complicated. In addtion, the bobbin is formed by comparatively expensive material such as plyethylene, polybutylene-terephthanol, phenol and the like, and so provision of the protecting legs means to increase the manufacturing price of the conventional bobbin.